Melt butter in a large heavy saucepan over medium heat. Add onion and garlic and sauté until aromatic, about 3 minutes. Stir in curry powder, ginger, sea salt, cayenne and black pepper. Add coconut milk and water; bring to a simmer and cook for 2 minutes. Add shrimp, mango and sweet potato to saucepan; bring back to a simmer and cook for 4 to 5 minutes more or until shrimp are pink. Garnish the shrimp and mango curry with green onion and cilantro.